During my playing i met one guy near town Sumpan - mutant medicus with name like Yummi. When i defeat him he told me about some lag and bla-bla-bla. He offered to fight one more time and Then (i won again) he promised to send bullet in my back and disappeared. When i met him second time(today) i defeated him once (seems he didnt remember me )and then choosed not to fight again. But he disappeared again What should i do with this event?

Tryed to make screenshot, maybe even did, but dunno where they are placed by the devs.

You will met him again and again if you wander around that same area (he is one of the random encounters there).
Basically there is a conversation and you have to choose the right options all the way through it. If you choose all the right ones maybe you will have a new team member at the end of the conversation

For screenshots you can either set the game to windowed mode and use Print Screen button or in Steam you can enable steam community ingame and use the Screenshot shortcut key shown there (Steam->settings->IN-GAME tab->click enable Steam community ingame, and make sure the Screenshot shortcut key field has a key in it, default is F12). the screen shots taken via steam can be viewed by choosing view->screenshots in steam

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When things may seem easy, have no fear, I can make them really complicated...really fast ;)

Once you get to that folder mentioned above, (C:\Program Files (x86)\Steam\userdata\<your user id>\760\remote\42170\screenshots), you have to know what the SteamAppID is for the particular game is that you want to find....

Easiest way I've found is to take a screenshot in the game and open up the overlay so that you see your screenshot's thumbnail. If you click on the screenshot and then click the button Show on Disk, you'll see the appid in the address bar at the top of the screen (assuming you have that function enabled in Windows Explorer; if not, then that's a whole different "fix"). If you don't see the file address, you can sort the remote folder window by clicking on the Date Created (or sometimes the Date Modified column if you've had the game installed awhile) in Descending order and you should see your game at or near the top. Once you find it though, you can create a shortcut on your desktop to get back to it easily.